The invention relates to a method for regulating the roll gap pressure of a roller press and to a roller press corresponding thereto.
Roller presses which consist of two—as a rule—identically sized, rotatably mounted rollers which run in opposite directions, rotate at the same circumferential speed and form a narrow roller gap between them, are frequently used for crushing or compacting granular material. The material to the crushed or compacted is pulled through said roll gap, the granular material being crushed or compressed under the high pressure prevailing in the roll gap. The result of said treatment, namely crushing or compacting, is for the main part dependent on the material characteristics of the granular material to be crushed. The crushing process in the roll gap described here was described for the first time as high-pressure crushing by Schonert et al. in German Disclosure DE 27 08 053 A1 and since then it has been applied as a genus of the types of crushing along with grinding by means of cutting and breaking.
Along with the pressure in the roll gap, high-pressure crushing calls for a plurality of parameters to be maintained in the roller press used for an optimum, low-energy and low-wear crushing process. For example, it is important for the rollers of the roller press used to rotate without relative slippage so that the rollers, as a result of the grinding material moving in a shearing manner, do not grind but press exclusively. In addition, it has been shown that the correct amount of fresh material supplied per unit time to the roll gap of the roller press used also plays a considerable roll for the optimum operation of the roller pressure used. If the roll gap is provided with too small an amount of fresh material per unit time, the roller press operates as a breaker, in particular when using rollers equipped with hard reinforcing bodies, the granular material to be crushed as fresh material being broken by point loads. Said type of crushing is less energy-efficient than high-pressure crushing and it does not result in the desired fine product. If, contrary to this, the roll gap is provided with too large an amount of granular material as fresh material per unit time, the grinding material, produced by fresh material and circulating material, is compressed too strongly in the roll gap such that enclosed air is not able to escape and the roll gap of the roller press used tends to get really blocked up. The resiliently mounted rollers deflect in this case, the fresh material, present in excess, falls uncrushed through the roll gap and the roller press then operates in the previous state again until it has to deflect repeatedly in order to allow the fresh material, present in excess, to pass through the roll gap. The roller press thus moves into a first type of oscillatory motion alongside other oscillatory motions and it begins to vibrate mechanically.
Along with said type of mechanical oscillation, which is generated as a result of the rollers creeping forward and backward in their resilient bearing arrangement at a frequency which is high compared to the moved masses, there is a further oscillatory motion inside the roller press in the form of an oscillatory motion of the rollers which is generated on the rotating rollers by the repeated, braking action of the over-filled roll gap. As a result of said rhythmical braking which is brought about by an over-filled roll gap and renewed acceleration by the drive, the rollers move into a rotational oscillation where the moment and the angular speed of the roller fluctuate in a steady manner. In particular, this is the case with driven rollers when a roller press has only one driven roller with a co-rotating roller.
Particular types of oscillatory motions can be generated when the overload with too much fresh material occurs only in one part of the roll gap. The rollers can then exhibit a combined oscillation which consists of a forward and backward movement of the rollers in the horizontal direction at right angles to the extension of the roll gap and of a rotational oscillation. In this case, the rollers can also run through a slight, oscillating change in position where the respective roller carries out a rotation about a vertical axis by very small angular amounts. In the case of said movement, the roller is not displaced evenly with both bearing blocks that support it, but rather the two bearing blocks change their position alternately with respect to one end each of a roller.
Mechanical oscillatory motions of very short duration and high frequency and amplitude in the form of an impact are also generated during the passage of pieces of fresh material which are too large or during the passage of constituent parts which are not crushable by high-pressure treatment in the roll gap, such as, for example, metal pieces, that is hammer heads, large steel rivets or bolts, digging teeth or other unwanted metal parts, which are situated in an unwanted manner in the fresh material and are able to pass in an undesirable manner into the fresh material when the raw material is dismantled.
In addition, mechanical oscillatory motions can also be generated inside a roller press during the operation to start-up the roller press when the grinding material is not yet circulating in a balanced manner or the circulating material has a composition which is not yet balanced. Finally, mechanical oscillatory motions are also generated when fresh material which is wet and fine-grained is used.
If the frequency of an aforementioned mechanical oscillatory motion accidentally reaches the frequency of a natural oscillation of the roller press, with every individual oscillatory motion more energy is transmitted to the entire system of the roller press, as a result of which serious damage can be caused to the bearings, the roller surfaces and other components of the roller press as a whole, not least of all consequently because the rollers can reach an individual weight of in excess of 70 t and an oscillating mass of said order of magnitude presents very great challenges to even very sturdy machine frames.
Naturally, the entire system of the roller press is damped mechanically as a result of its design. The damping is provided on the one hand by the hydraulic system in which the hydraulic fluid flows back and forth at high speed through the lines, which are fine compared to the diameters of the hydraulic plunger or cylinder, and damps very strongly as a result. In addition, the movement of the bearing blocks along the slide rails of the loose rollers also absorbs a high mechanical energy in the form of friction, as a result of which an oscillatory motion is damped.
However, insofar as the roller press moves into an unwanted oscillatory mode it is shown that the roller press no longer operates in an energy-efficient manner and over and above this is also strongly loaded mechanically.
In order to avoid or to prevent mechanical oscillatory motions being realized at all in the roller press, generated by over-loading the roll gap with fresh material, the amount of fresh material discharged per unit time can be regulated by, for example, less fresh material per unit time being put onto the roll gap by the discharge apparatus when an unwanted oscillatory motion is detected in the roller press. However, the disadvantage of this is that a comparatively long reset time for the regulated section from the controlled feed apparatus up to the detected oscillatory motion has to be accepted. A certain time passes until the modified feeding of the roll gap operates with fresh material and finally the oscillatory motion is reduced as a result. Up to that point, considerable damage can have been caused to the roller press or can accumulate when this type of regulating intervention is necessary more frequently.
The following measures from the prior art are known for monitoring the functioning of crushing apparatuses:
Printed document US2010/0102152A1 describes conical breakers which are provided with approximation sensors such as, for example, ultrasound sensors or laser sensors. By measuring the width of the outlet gap, the width of the gap can be adapted to the process conditions by means of lifting or lowering the cone, as a result of which uneven rotations which can damage the cone are avoided.
US2004/0255679A1 describes a rotary drum grinder for crushing minerals, said rotary drum grinder having an acoustic sensor in the drum by way of which it is possible to detect loads on the drum that are too heavy, e.g. caused by solid rock.
DE10132067A1 discloses a method for acoustically monitoring threatening operating states, e.g. slippage, in cylinder mills. To this end, the noises occurring in the cylinder mill, e.g. the noise level, are detected by way of a microphone and the frequency spectrum is evaluated.
However, none of the printed documents disclose how said unwanted operating states can be avoided or eliminated.
It would consequently be desirable if a roller press were to be able to be operated in such a controlled manner that the mechanical oscillatory motions do not take place. Consequently, it is the object of the invention to operate a generic roller press such that a mechanical oscillatory motion does not occur.